1. Field of the Invention
Exemplary embodiments of the present invention relate to radar, and, particularly, to an apparatus and a method for detecting a target in near field to measure a range and a relative velocity of a target in near field using a microwave or a millimeter wave having a frequency modulated continuous waveform.
2. Description of Related Art
When a target of which the relative velocity is Mach x.0 to Mach y.0 and the radar cross section (RCS) is equal to or more than −00 dBsm encounters with a target detecting device (TDD), the target detecting device (TDD) needs to detect the target and generate a detonation signal and needs to generate the detonation signal by a self-destruct (SD/ED) signal which is generated only if a missile is abnormally operated.
To this end, the TDD is largely configured to include a transceiver, an antenna unit, and a signal processing unit, and uses x pairs of transmitting and receiving antennas on a surface of a projectile in a circumferential direction to transmit and receive electromagnetic waves and when a target is present in near field, allows the signal processing unit to analyze the received signals from the antennas so as to detect the target.
Comparing a binary phase shift keying (BPSK) modulation scheme to a frequency modulated continuous wave (FMCW) modulation scheme in order to detect the target in near field under the situation in which a sea clutter having the RCS relatively larger than that of the target is present, since the FMCW modulation scheme has a relatively larger range rejection level to remove the unwanted sea clutter out of a range and easily acquires various types of information on the target, the FMCW modulation scheme is used.
Generally, since a frequency modulated continuous wave (FMCW) proximity sensor radar requires relatively low power, a small detection bandwidth, and the like, the FMCW radar may be miniaturized, used in near field (tens of m to hundreds of m), have high range resolution, and the like.
In particular, since a millimeter wave sensor may be small and lightweight and have high resolution and accuracy, the millimeter wave sensor is used to measure a range and/or a velocity, such as autonomous intelligent cruise control (AlCC), industrial level gauge, imaging, and indoor presence detection.
To detect the target in near field under the situation in which the sea clutter having the large RCS is present, the received power (Pr-target) of the target and the received power (Pr-clutter) of the sea clutter are calculated depending on a radar Equation and are compared with each other.
                              P                      r            -            target                          =                                            P              t                        ⁢                          G              t                        ⁢                          G              r                        ⁢                          λ              2                        ⁢            σ                                4            ⁢                                                  ⁢                          π              3                        ⁢                          R              4                                                          Equation        ⁢                                  ⁢        1            
                              P                      r            -            target                          =                                                            P                t                            ⁢                              G                t                            ⁢                              G                r                            ⁢                              λ                2                            ⁢                              σ                0                            ⁢                              θ                B                            ⁢                              ϕ                B                            ⁢                              csc                ⁡                                  (                  Ψ                  )                                                                    256              ⁢                                                          ⁢                              π                2                            ⁢                              R                2                                              ⁢          η                                    Equation        ⁢                                  ⁢        2            
In Equations 1 and 2, Pr: received power, Gt: transmitting antenna gain, Gr: receiving antenna gain, λ: wavelength, σ: target RCS, σ0: clutter RCS per unit area, (θB, φB): antenna beam width, ψ: Grazing angle, R: range, η: antenna overlap ratio.
FIG. 1 illustrates the received power of the target and the received power of the sea clutter which are calculated. In Xm or more, when the RCS of the sea clutter is equal to or more than σ0=−00 dB, it may be appreciated that the received power of the sea clutter is larger than that of the target at all times. Therefore, the range rejection level to remove the unwanted clutter out of the range is, an important factor for selecting a transceiver type and requires the rejection of 30 dB or more.
Further, referring to FIG. 2, the FMCW modulation scheme performs frequency modulation on a continuous wave signal to extract an intermediate frequency fif (Hz) which is a difference frequency (|ft−fr|) between a signal ft (Hz) 210 transmitted to the target and a received signal fr (Hz) 220 received by being reflected from the target, thereby measuring the range and the velocity of the target. The transmitted signal of FIG. 1 is modulated into a triangular wave and has a period Tm (sec) and a frequency modulation bandwidth ΔF (Hz).
The received signal is delayed as much as a delay time τ=2R/C (sec) corresponding to a range R(m) propagated after being reflected from the target and suffers from Doppler shift fd=2Vr/λ as much as the relative velocity Vr to the target. Therefore, the intermediate frequency fif(Hz) is calculated depending on the following Equations 3 and 4.
                              f                      if            -            upsweep                          =                                            f              b                        -                          f              d                                =                                                    4                ⁢                Δ                ⁢                                                                  ⁢                                  Ef                  m                                ⁢                R                            C                        -                                          2                ⁢                                                                  ⁢                                  V                  r                                            λ                                                          Equation        ⁢                                  ⁢        3            
                              f                      if            -            downsweep                          =                                            f              b                        +                          f              d                                =                                                    4                ⁢                                                                  ⁢                Δ                ⁢                                                                  ⁢                                  Ef                  m                                ⁢                R                            C                        +                                          2                ⁢                                                                  ⁢                                  V                  r                                            λ                                                          Equation        ⁢                                  ⁢        4            
In Equations 3 and 4, fm is a modulation frequency, C is a propagation velocity on a free space, and λ is a signal wavelength. fb is proportional to the range R and fd is proportional to the relative velocity Vr.
A beat signal fb (Hz) and a Doppler frequency fd (Hz) from the above Equations 3 and 4 may be calculated depending on the following Equations 5 and 6.
                              f          b                =                                                            f                                  if                  -                  upsweep                                            +                              f                                  if                  -                  downsweep                                                      2                    =                                    4              ⁢              Δ              ⁢                                                          ⁢                              Ff                m                            ⁢              R                        C                                              Equation        ⁢                                  ⁢        5            
                              f          d                =                                                            f                                  if                  -                  downsweep                                            +                              f                                  if                  -                  upsweep                                                      2                    =                                    4              ⁢                              V                r                                      λ                                              Equation        ⁢                                  ⁢        6            
That is, when the intermediate frequency fif-upsweep at the time of the up-sweep depending on Equation 3 and the intermediate frequency fif-downsweep at the time of the down-sweep depending on Equation 4 are detected and the fb and the fd are derived from the fif-upsweep and the fif-downsweep depending on Equation 5 and Equation 6, since the frequency modulation bandwidth M, the modulation frequency fm, and the wavelength λ are preset values, the range R(m) and the relative velocity Vr of the target may be obtained.
However, leakage signals from various paths, such as an antenna mismatch of the transmitted signal, an inter-antenna crosstalk, and a leakage on the substrate generate frequency components or DC components having a low intermediate frequency band like an actual target. The leakage noise signals in near field may not be separated from a target signal on a frequency, and since a size of the leakage noise signals is relatively larger than that of the reflected signal from the actual target, a detection blind zone in near field occurs, such that the range and/or the velocity of the target may not be measured within a very short range.